Optical flow cell incorporating automatic self-cleaning

ABSTRACT

Optical flow cells used, for example, for making scattered light measurements on liquid borne samples, are often affected by particulate materials that adhere to the internal optical surfaces. These contaminating particulates can cause distortions of the scattered light signals to be measured from the illuminated samples within the flow cell. Such particulates are difficult to remove, especially while the cell is assembled. A method for dislodging and removing such particulate contaminants is described that consists of attaching externally to the flow cell an ultrasonic generator whose frequency of operation is swept initially over a range that would couple most effectively with the internal structures of the flow cell itself. The optimal frequency is then selected automatically by monitoring the power dissipated and fixing the ultrasonic frequencies of operation at those values corresponding to regions of maximum power dissipation. Such ultrasonic coupling must be accompanied by an impressed flow through the cell that can remove from the cell particulates that have been dislodged.

This is a continuation-in-part of application Ser. No. 09/523003 filedMar. 10, 2000 still pending entitled “A self cleaning optical flowcell”.

PRIOR RELATED PATENTS AND APPLICATIONS

The present invention is directed to an optical flow cell design thatcan be cleaned automatically without invasive physical means. It isparticularly useful when optical measurements are made followingchromatographic separation since these are often associated withparticulate material and air bubbles that tend to adhere to the opticalsurfaces themselves.

Expressly incorporated herein are the following patents and applicationsconcerning flow cells and related structures whose performance would beimproved with the new invention.

U.S. Pat. No. 4,616,927—“Sample Cell for Light Scattering Measurements,”(Oct. 14, 1996)

U.S. Pat. No. 4,907,884—“Sample Cell Monitoring System,” (Mar. 13, 1990)

U.S. Pat. No. 5,305,071—“Differential Refractometer” (Apr. 19, 1994)

U.S. Pat. No. 5,404,217—“Laser liquid Flow Cell Manifold System andMethod for Assembly,” (Apr. 4, 1995)

U.S. Pat. No. 5,530,540—“Light Scattering Measurement Cell for VerySmall Volumes,” (Jun. 25, 1996)

Application Ser. No. 08/989,364 “A New Electrode Design for ElectricalField Flow Fractionation”, Steven P. Trainoff. (Filed Dec. 12, 1997)

Application Ser. No. 08/870,937 “Extended range interferometricrefractometer”, Steven P. Trainoff, David T. Phillips, Gary R. Janik,and Douglas W. Shepard. (Filed Jun. 6, 1997)

BACKGROUND

In the field of light scattering, as applied to determine the molar massand mean square radius of solvated molecules, measurements are made fromsolutions comprised of a solvent containing a dissolved sample. Bymeasuring the scattered light variation with scattering angle andmeasuring the concentration of the solute, one may in principledetermine the molar mass and mean square radius of such solvatedmolecules. Similarly, the light scattering properties of sub micrometerparticles in liquid suspension may be used to determine their averagesize Light scattering techniques may be applied as well for measurementsinvolving inelastic light scattering such as photon correlationspectroscopy, Raman spectroscopy, fluorescence, etc. These measurements,usually performed at a fixed single angle, are used to determine thehydrodynamic size of the particles or molecules illuminated.

Light scattering measurements are often made with a light scatteringphotometer wherein the sample is introduced into an optical cell such asreferenced above in U.S. Pat. Nos. 4,616,927 and 5,404,217. Interferingwith such optical measurements are a variety of contaminants whosepresence inside the flow cell often contribute to the recorded lightscattering signals in such a manner as to distort or even mask them Suchcontaminants arise from various sources, many of which cannot beavoided. Included among these are small air bubbles, fine particlesshedding from chromatographic columns if such are being employed toseparate the molecules or sub-micrometer particles nor to measurement,aggregates formed from the sample itself which may have a strongaffinity for the internal optical surfaces, contaminants in the poorlyprepared solvent, debris from previous measurements that build up on theoptical surfaces, etc.

During the measurement process, the presence of these contaminants areoften recognized indirectly through the effects they have on thescattering or are noticeably visible through physical examination of thescattering cell, or both. There are various means by which suchcontaminants are removed or dislodged from the internal optical surfacessuch as flushing the optical cell with different solvents such as acidsor detergents or introducing a large air bubble in the manner of thefamiliar Technicon AutoAnalyzer of the 1960s. Sometimes, no matter howmuch effort has been expended, the flow cell must be disassembled andeach component cleaned manually. Once disassembled, one of the mostuseful means for cleaning surfaces is to use ultrasonic waves as createdin an ultrasonic cleaning bath. The components are placed in a fluidsuch as water and ultrasonic waves whose fixed frequencies are of theorder 50 kHz are propagated throughout the bath These waves aregenerally generated by means of piezoelectric transducers well coupledto the bath chamber. At the frequencies and power levels traditionallyapplied, cavitation effects generally cause the generation of bubbleswhich, when driven against a surface, tend to assist in the cleaning andscrubbing of such surfaces.

Although the disassembly of an optical cell and the subsequent cleaningof its parts in an ultrasonic bath are effective, it is time consuming.Unfortunately, it is often the only means possible. When the opticalcell is used in a high temperature environment, such as is the case forchromatographic separations requiring high temperature solvents, thetraditional disassembly concept becomes even more time-consuming sincethe temperature of the chromatograph itself must often be reducedsignificantly to obtain access to the optical cell, which is thenremoved and cleaned. High temperature chromatographs, and especially thecolumns used therein, can be damaged during temperature cycling, which,therefore, must be carefully executed. The process of cleaning aninternally mounted optical cell can, in such a case, require up to 24hours to effect a removal, cleaning, and reinstallation.

It always has been thought desirable to have optical elements of thelight scattering cells designed in such a manner as to prevent thedeposition of extraneous materials on their surfaces or, at the veryleast, design them in such a manner as to permit the cleaning of theirinternal surfaces with minimal effort. To this end, many structuresrequiring clean, particulate-free surfaces have been designated as“self-cleaning” such that once internal precipitants are detected theymay be removed without need for disassembling the structures themselves.A process by which the initial formation of such contaminants may bereduced is taught, for example, by Davidson in U.S. Pat. No. 5,442,437wherein windows, through which optical measurements are to be made, areso positioned that they extend into the flowing solution which, thereby,continuously “. . . scour said window to minimize contamination andclouding [thereof] . . . ” This, of course, is an old concept that wasdisclosed in U.S. Pat. No. 4,616,927, referenced above, and numerousother similar implementations whereby it is necessary to cleanobservation windows of various types. Although such cleaning may keepthe observation windows clear of particulate debris for some time,eventually sufficient particles may accrete so as to interfere withlight passing through some optical surface.

Another example of a self cleaning cell is U.S. Pat. No. 4,874,243 byPerren wherein the windows are at an angle to the direction of flowwhich results in a “. . . self cleaning action . . . ” as the flowingstream passes over them. A similar example is U.S. Pat. No. 4,330,206 ofGausmann et al. wherein is shown a measurement chamber “. . . inherentlyself-clearing of air or gas bubbles in liquid samples . . . [whichprovide] inherently efficient cleansing of the measurement chamber . . .” This is achieved by outlet means lying above the optical regionguiding thereby air bubbles up and out of the fluid enclosed. The fluidflowing into the measurement channel strikes the cell window obliquely,thus cleaning it and maintaining it free of contaminants.

Berger in his U.S. Pat. No. 4,496,454 describes another example of aself-cleaning mechanism for the case of electrochemical cells used withcertain forms of liquid chromatography. His invention attacks a similarproblem for electrochemical detection that faces light scatteringdetection: the fouling of the electrode surfaces during measurementwhich, in turn, affects the detector response. In the light scatteringcase, the optical surfaces can become fouled with particulates and smallair bubbles. Berger achieves his cleaning by using a capillary tube togenerate a water jet perpendicular to the detector electrode surface

In addition to such fluid cleaning means described above, there exist anumber of mechanical means exemplified by Wynn in his U.S. Pat. No.5,185,531. In Wynn's implementation, the optical windows are kept cleanby introducing periodically mechanically controlled flexible wiperblades “. . . extending from opposite sides of [a] . . . blade holderfor wiping engagement with the window surfaces . . . ” Otherimplementations of a wiping motion to clean an optical cell may be foundin U.S. Pat. No. 3,844,661 by Birkett et al. or U.S. Pat. No. 4,074,217by Yanagawa.

Although the use of ultrasonic waves appears an attractive means forremoving particulates from surfaces, such as described by Neefe in hisU.S. Pat. No. 4,457,880, it has never been used as a component of anoptical cell to permit self cleaning action. There are three basicreasons for this omission. First is the fact that there has been neithermeans for establishing a proper frequency regime to achieve suchcleaning nor means for localizing the cleaning action to the internalcell surfaces that require it. Secondly, even were such a self cleaningdevice integrated with the cell structure, there could be no assurancethat, once removed from the internal cell surfaces, the particulateswould not re-adhere or simply remain within the cell to adhere later tosome other region. Finally, traditional ultrasonic waves used incleaning are generated at frequencies of the order or 50 kHz, which, atthe power levels traditionally employed and in fluids such as water,induce cavitation effects that result in the generation of bubbles. Suchbubbles are most helpful because of their implied scrubbing action onthe surfaces to be cleaned. Were such bubbles generated within anoptical cell, the bubbles themselves could be expected to adhere tosurfaces within the fine interstices of such cells defeating, thereby,the cleaning concept ab initio.

Ohhashi in his U.S. Pat. No. 4,672,984 extends the ultrasonic conceptfor cleaning optical surfaces by providing a plurality of cleaningsteps, each of which may involve a different working liquid and/orultrasonic intensity applied over varying periods of time. Once again,such cleaning is done externally to any enclosed structure with theparts to be cleaned transported individually to the array of cleaningbaths. He does not discuss the frequency of the applied ultrasonicfrequencies nor any possible variations thereof, so one assumes that heemploys the standard cavitation prone frequencies around 50 kHz,

Honda et al. in their U.S. Pat. No. 5,656,095 introduce the concept ofmultiple frequencies, some of which are applied intermittently todestroy the bubbles generated by the continuously applied frequency.Such an action results in corresponding pressure pulses to which isattributed a “. . . greatly improved . . .” washing effect. Theyconsider so-called low frequency generation as occurring at frequenciesof 28 kHz, 45 kHz, and 100 kHz whereas high frequency generationdescribes generation at 160 kHz. The high frequency ultrasonic waves aresaid to generate bubbles in the size range of 20 μm to 500 μm while theintermittent low frequency waves destroy the bubbles, generating as theycollapse, even higher orders of ultrasonic waves.

The present invention is concerned with the implementation of anultrasonic cleaning device that is integrated with an optical flow celland controlled in such a manner as to permit sonic coupling with thoseinternal regions of the cell most needed to be particulate free. Sonicwaves are used in a manner by which cavitation is avoided wheneverpossible since such cavitation can cause etching or other damage tofinely polished optical surfaces.

SUMMARY OF THE INVENTION

This invention presents a new design concept for the cleaning of opticalsurfaces within flow cells used in conjunction with light scatteringmeasurements such as commonly employed in the field of analyticalchemistry and, more particularly, for liquid chromatography. Basic tothis invention is the incorporation into the flow cell structure itselfof means to provide internal to the flow cell extremely high frequencysonic waves such as would be produced by means of an electrically drivenpiezoelectric transducer. The frequencies of these waves are muchgreater than those employed by Honda et al. In order to avoidcavitation, yet be in resonance with the typical internal dimensions ofthe cleansed flow cells, frequencies of the order of 1 MHz/sec areemployed. There are many different types of flow cells for which thisdesign would be useful including those referenced above. In B. Chu'stextbook on “Laser light scattering”, a number of additional designs maybe found; though these are by no means exhaustive.

Key to this invention are four features: 1) integrating, by goodmechanical contact means, the sonic source, a piezoelectric transducerin the preferred embodiment, and the optical flow cell; 2) varying thefrequency of the applied ultrasonic waves so as to couple well withthose internal regions where the dislodgment of particulates isrequired; 3) using frequencies of the order of a MHz which are muchgreater than those traditionally used for ultrasonic cleaning purposesand, at practical power levels, beyond the frequencies thatconventionally would cause cavitation in most liquids; and 4) providinga flowing fluid means during the application of the ultrasonic waves bywhich dislodged particulates may be removed from the cell.

Although such an integrated cleaning technique may be applied to staticoptical cells that are not generally operated in a flow through mode,when used with such mechanically coupled ultrasonic waves, means must beprovided to permit a flow stream to remove particles dislodged by saidsonic cleaning during the application of said ultrasonic waves.

The requirement that the frequency of the applied sonic waves must beadjustable, so as to couple the sonic energy most efficiently to theinternal regions of the flow cell structure most prone to the presenceof unwanted particulates, may equally well be served by automatically,and repetitively, scanning a range of frequencies that includes thosebest suited for the internal regions to be cleaned. Note that atfrequencies of the order of 1 MHz in water, the associated wavelengthsare of the order of 1.5 mm, approximately the diameter of the flow cellof the flow cell of U.S. Pat. Nos. 4,616,927 and 5,404,217 and relatedstructures. The dislodgment of particles by the present inventive meansrelies upon mechanical displacement by the ultrasonic waves themselvesrather than the more traditional scrubbing action created in largemeasure by the cavitation created air bubbles turbulently bombarding theaffected surfaces.

The fluid that must be flowing through the flow cell structure duringapplication of the ultrasonic waves throughout the structure must be initself particle-free. When applied to a flow cell in conjunction with achromatographic separation, this fluid would correspond to the so-calledmobile phase of the chromatographic separation process. Such fluidsshould be free of particulates and are often degaussed and filteredprior to use in the chromatograph. Additionally, since the ultrasonicfield can induce particle aggregation within the flow cell, theresulting aggregates are more easily flushed from the flow cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a flow cell similar to the type disclosedin U.S. Pat. No. 5,404,217.

FIG. 2 shows a top view of the cell of FIG. 1 providing a port throughwhich may be observed the illuminating laser beam and bore.

FIG. 3 shows a top view of a piezoelectric transducer coupled to a flowcell structure, which permits direct observation of particulatestherein.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an exploded view of the key elements of a flow cell of thetype disclosed in U.S. Pat. No. 5,404,217. A manifold comprised ofelements 1, 2, and 3 hold a glass cell 4 through which is a bore 5. Ateach manifold end is a glass window 6, 7 suitably sealed by O-ring means8 and locking fixtures 9. Fluid, containing solvated molecules orentrained particles enters through fitting 10 and exits cell through 11.An illumination source, usually a focused beam 12 from a laser 13,enters through window 6. This figure shows a characteristic flow cellStructure containing many internal surfaces and regions capable oftrapping particulates or permitting precipitates to form thereon. A topview of this cell is shown in FIG. 2 providing a port 14 through whichthe laser beam 12 and bore 5 may be observed. In the event there areparticles IS present on the walls of the bore 5, often they may bevisually observed appearing as bright sources of light. Accordingly, itis an objective of this invention to provide a means by which suchextraneous light sources, arising from particulates affixed to the cellwalls, may be removed from this type of flow cell as well as any otherstructures wherein such particulates may become affixed.

In the present specification, the term “flow cell” is used to describe astructure comprised of the glass cell itself, the windows through whichthe incident beam of light enters, and all supporting and ancillaryelements such as the various pieces of the manifold shown in FIG. 1.Although the light source for the preferred embodiment of this inventionis generally referred to as a laser, the invention applies equally wellto other types of optical flow cells where their light source may befrom incandescent lamps, light emitting diodes, arc lamps, etc., or eveninternally generated by constituents of the sample itself

A preferred embodiment of the invention is shown in FIG. 3 wherein apiezoelectric transducer 16 is maintained in mechanical contact with theflow cell 17 by illustrated means as follows; direct contact plate 18 towhich said piezoelectric transducer 16 is bonded, electricallyconductive spring 19 compressed against said transducer by washer means20 which distributes pressure evenly as imparted from spring washer 21,and threaded retainer 22 which hold assembly within housing module 23.The assembly housing 23 is mechanically attached to the read head 24,holding the flow cell 17, by bolt means 25. Power is supplied to saidtransducer via power connector means 26.

The preferred embodiment, just described, provides for firm mechanicalcontact with the flow cell of the contact plate 18 to which thepiezoelectric transducer 16 is attached. The mechanical contact isachieved by pressure means imparted to piezoelectric transducer/contactplate via compression of the spring washer 21 and conductive spring 19by the compression occurring as the threaded retainer 22 is threadedinto the assembly housing 23. The housing 23 may include a stoppingmeans whereby a limiting compression may be set. Alternatively, saidpiezoelectric transducer may be attached directly through bonding orother affixation means including gluing or cementing using epodes orother adhesives. Obviously, there are many other locations on any givenflow cell structure where such transducer device may be attached to makegood mechanical contact for use subsequently to generate sonic wavespermeating throughout said flow cell bore and other internal regionswherein particulates may form or become attached.

The concept of attaching a piezoelectric transducer directly to asurface for purposes of removing particulates is not new. For example,Collier in his U.S. Pat. No. 5,724,186 shows how such an attachment oftwo piezoelectric transducers, in a so-called bi-morph configuration,can provide a means for clearing a vehicular rear view mirror of waterdroplets. However, the concept of attaching an ultrasonic transducer toa structure for purposes of cleaning inaccessible internal enclosedregions is new and unique. Note that the particles of Collier'sinvention are limited to water droplets which must be in an airenvironment with the mirror face essentially parallel to the earth'sgravitational field.

In the preferred embodiment of an electronic driving circuit forpowering the ultrasonic transducer, of the type exemplified by apiezoelectric transducer, it initially should generate sonic wavesspanning a broad swept range of frequencies. Since it is not generallypossible to predict exactly the frequency that would couple best withthe particular internal regions of the flow cell structure whereinaffixed particles would be loosened therefrom by the corresponding sonicwaves, the preferred embodiment of this invention allows for thesweeping of the excitation frequency generated by the piezoelectrictransducer. However, by including a sensor to monitor the powerdissipated by the Piezoelectric circuit as the frequency is scanned, thesystem could be tuned automatically to the frequencies that areaccordingly best coupled to the structural features of the scatteringcell. Each internal region will have an associated range of frequenciesbest coupled for purposes of dislodging particulates and by searchingfor those frequencies at which power dissipation is a maximum, thesebest frequencies may be selected automatically. In this mode, changes insolution properties, that might require a slightly different optimalfrequency from that used with a different solution, may be automaticallycompensated, obviating the need to make a manual tuning correction. Wehave found that the optimal range within which the best-coupledfrequency should be selected automatically should be between about 0.5MHz and 5 MHz for the structures such as shown in FIG. 1.

Although the preferred embodiment of this invention suggests thatextremely high intensity ultrasonic waves be employed operating at themegahertz range so as to couple more effectively with the internalelements of the optical flow cell, this is certainly not the first timethat such frequencies have been employed for cleaning purposes. TheBranson Ultrasonics Corporation of Danbury Conn., for example, offersfor sale its 400 kHz MicroCoustic® device capable of cleaning “ . . .irregular geometries, tight clearances and highly finished surfaces . .. ” by non-cavitational means. However, there is no variation offrequency nor is the device integrated with the object to be cleaned. Itis representative of the traditional immersion bath methods, thoughoperated at a higher frequency. Again, all surfaces to be cleaned areexternal surfaces, though the Branson concept emphasizes the cleaning ofsurfaces containing very fine features. Accessibility of the ultrasonicwaves to these fine features requires that these surfaces be placedwithin baths providing direct exposure to said ultrasonic waves. Thepossibility of coupling external sonic sources to a structure whoseinner surfaces contained fine features to be cleared of adheredparticles was never considered for possible application of the Bransonultra high frequency devices. This is because the Branson devices andsimilar devices manufactured by others are designed to clean byultrasonic means a broad range of parts which do not include parts andsurfaces internally situated with respect to complex structures such asoptical flow cells.

The dislodgment of particles by the present inventive means relies uponthe mechanical displacement by the ultrasonic pressure waves themselvesrather than the more traditional scrubbing action created in largemeasure by the cavitation-created air bubbles turbulently bombarding thesurfaces to be cleaned of particles. Cavitation induces the dissolutionof gas from the fluid and this can result in bubbles, which, like anyother foreign particulates present in the optical cell, are inimical tothe performance of light scattering measurements where they mayinterfere with the scattered or incident light. Note that at frequenciesof the order of 1 MHz in water, the associated ultrasonic wavelengthsare of the order of 1.5 mm, approximately the diameter of the flow cellof U.S. Pat. Nos. 4,616,927 and 5,404,217 and related structures. Suchwaves may propagate longitudinally and throughout the flow channelsproducing pressure fluctuations both transverse and parallel to theoptical surfaces thereon. Operating the ultrasonic piezoelectrictransducers at conventional power levels and ultrasonic frequencies ofthe order of 50 kHz would generally result in the creation of additionalgas bubbles further contaminating the flow cell and optics. However, atsufficiently low power levels for most fluids, such cavitation effectscould be minimized, at the expense of cleaning efficiency.

During experiments with the inventive concept, it has been noted thatalthough the sonic waves effectively dislodge the particulates from theoptical regions within typical flow cells, these same particulates aredriven to other proximate regions where they again become affixed.Particulates were seen also forming aggregates with other particles,such aggregates being caused by the impressed ultrasonic fields. Thisself-scavenging effect further helps collect dispersed particles as theapplied flow stream more easily drives out larger particulates becauseof their greater cross section. In order to drive them out of the flowcell, it is essential that a particle free flow be directed through thecell during the ultrasonic dislodgment process. In this manner, theparticulates are forced to progress toward the cell outlet whileexecuting a somewhat random walk from one region of the cell surface toanother. Even in the presence of such an imposed flow, particles areoften observed to move against the stream and become re-affixed upstream. However, these are but statistically random motions, which arethen superimposed upon the steady stream flow resulting in theireventual removal from the flow cell. The total time required to clearthe cell of FIG. 1, for example, is of the order of a minute. Thus it isnot necessary that the impressed sonic cleansing action be alwaysfunctioning. Its activation is, therefore, generally controlled by theoperator of the light scattering apparatus on the basis of his/herobservation of the light scattering signals being collected. Naturally,such periodic cleaning could be programmed to occur automatically usingsuch light scattering signals and establishing therefrom the criteriaindicative of the presence of particulate contaminants.

The imposed fluid flow through the flow cell structure duringapplication of the ultrasonic waves throughout the structure must be initself particle-free. When applied to a flow cell used for making lightscattering measurements following chromatographic separation, this fluidwould correspond to the so-called mobile phase. Such fluids should befree of particulates and are often degaussed and filtered prior to usein the chromatograph.

For various types of optical cells wherein static or dynamic lightscattering measurements are to be made and there is no other source ofcontinuously flowing fluid to perform such flushing, it may necessary toattach or otherwise provide means by which such fluids may be introducedand removed from such cells in a continuous manner to carry out of saidoptical cells particles dislodged by the applied ultrasonic waves. Thisfluid itself, of course, must be free of particles and this usuallyrequires both prefiltering and degassing.

The application of the present invention for optical cells that are usedwithin chromatographs at elevated temperatures is a particularlyimportant one. As has been discussed earlier, the traditionaldisassembly and cleaning procedures become even more time-consumingsince the temperature of the chromatograph itself must often be reducedsignificantly to obtain access to the optical cell, which is thenremoved and cleaned. High temperature chromatographs, and especially thecolumns used therein, can be damaged during temperature cycling, which,therefore, must be carefully executed. The process of cleaning aninternally mounted optical cell can, in such a case, require up to 24hours to effect a removal, cleaning, and reinstallation. Theincorporation of the self-cleaning structure in such high temperaturechromatographs is, therefore, both desirable and essential. Thepreferred embodiment of the invention using a piezoelectric ultrasonicgenerator should be capable of operation at temperatures as high as 250°C.

A further problem that must be considered when such an implementation ofthe invention is employed concerns the ever-present fire dancers whenorganic solvents are used at both ambient and high temperatures. Sincethe ultrasonic circuitry requires application of voltages of the orderof 100 V, there will east the possibility of a spark-initiateddischarge. Accordingly, for such cases, it is important that a vapordetector (such as manufactured by Figaro USA, Inc.) be present in closeproximity to the ultrasonic transducer. The vapor detector can itself beused as a safety interlock to prevent operation of the transducerwhenever such a leak poses a fire or explosion danger.

Now whereas the most preferable embodiments and applications of the selfcleaning optical cell have been disclosed herein, it will be obvious tothose skilled in the art of optical measurements and preparing the cellsused therein that there are many obvious modifications and variations ofthe apparatus and method disclosed herein that may be implemented withequal effectiveness. All such modifications and variations areconsidered to be part of the invention.

What is claimed is:
 1. A method for automatically cleaning an opticalflow cell containing optical elements through whose surfaces light mustpass comprised of a) attaching an ultrasonic wave generator means infirm mechanical contact to said optical flow cell; b) selecting a rangeof ultrasonic frequencies best coupled to the internal regions of saidoptical flow cell through which light passes and where precipitates andaffixed particulates may occur; c) driving said ultrasonic wavegenerator means over said selected range of ultrasonic frequencies; d)monitoring the power dissipated by said ultrasonic wave generator; e)providing power monitoring means able to sense automatically thefrequency within said range of frequencies at which said powerdissipated is a maximum; f) selecting said maximum power dissipationfrequency within said range of ultrasonic frequencies and fixing saiddriving frequency at this value; and g) flowing a particulate free fluidthrough said optical flow cell while said attached ultrasonic wavegenerator has been fixed automatically at said maximum power dissipationfrequency.
 2. The method of claim 1 where said ultrasonic generator is apiezoelectric transducer.
 3. The method of claim 1 where said range ofultrasonic frequencies selected is between 0.5 and 5 MHz.
 4. The methodof claim 1 where said optical flow cell is the flow cell component of alight scattering photometer.
 5. The method of claim 4 where said lightscattering photometer is used in combination with a liquidchromatograph.
 6. The method of claim 1 where said particulate freefluid is the mobile phase used with a liquid chromatographic separation.7. The method of claim 1 where said firm mechanical contact is achievedby adhesive means.
 8. The method of claim 7 where said adhesive means isprovided by an epoxy material.
 9. The method of claim 1 where said firmmechanical contact is provided by spring pressure means.
 10. A selfcleaning optical flow cell comprised of a) an optical flow cell with allassociated support and mounting elements; b) an ultrasonic wavegenerator means attached in firm mechanical contact to said optical flowcell; c) a means for driving said ultrasonic generator means over aselected variable range of ultrasonic frequencies; d) power monitoringmeans able to sense automatically the frequency within said variablerange of frequencies at which power dissipated by said generator meansis a maximum; e) means to fix the driving frequency of said ultrasonicgenerator means at same frequency of maximum power dissipation; and f)means to flow through said optical flow cell a source of particulatefree and bubble free fluid while said ultrasonic generator has beenfixed automatically at said maximum power dissipation frequency.
 11. Theoptical flow cell of claim 10 where said ultrasonic generator is apiezoelectric transducer.
 12. The optical flow cell of claim 10 wheresaid variable range of frequencies is between 0.5 and 5 MHz.
 13. Theoptical flow cell of claim 10 where said optical flow cell is the flowcell component of a light scattering photometer.
 14. The optical flowcell of claim 13 where said light scattering photometer is used incombination with a liquid chromatograph.
 15. The optical flow cell ofclaim 10 where said particulate free fluid is the mobile phase used witha liquid chromatographic separation.
 16. The optical flow cell of claim10 where said firm mechanical contact is provided by spring pressuremeans.
 17. The optical flow cell of claim 10 where said firm mechanicalcontact is achieved by adhesive means.
 18. The optical flow cell ofclaim 17 where said adhesive means is provided by an epoxy material.